A light emitting diode (LED), a cold cathode tube, and an EL are employed as a luminous element for illumination, but the LED and the cold cathode tube are thick in width. Therefore, if these elements are employed in a display device or an input unit, the depth will become deep.
In additions it is difficult for these elements to be uniformly illuminant, depending on the shape and the size of the illuminant surfaces and consequently, an uneven illuminance sometimes arises.
For this reasons ELs are frequently used as a luminous element which is a thin type and which is evenly illuminant regardless of the shape and the size of the illuminant surface.
This EL, as shown in FIG. 1, is connected to a power supply (inverter) for lightening an EL which converts a DC power to an AC powers and the EL is lightened by an AC voltage of typically 200 to 400 V (P--P).
As one of the methods which individually switch the luminescence brightness of a plurality of ELs, there is a method which varies the input voltage of an inverter connected to each EL.
As shown in FIG. 2, if the input voltage of the inverter is made high, the amplitude of an AC voltage that is applied to an EL becomes large as shown in FIG. 3 and the EL is brightly luminant.
Conversely, if the input voltage of the inverter is made low, the amplitude of an AC voltage that is applied becomes small and the EL is darkly luminant.
In FIG. 4 there is shown the relationship between the input voltage of the inverter and the luminescence brightness of the EL.
In FIG. 2, since the input voltages of the inverters connected to an EL1 and an EL3 are higher than the input voltage of the inverter connected to an EL2, AC voltages that are applied to the ELI and the EL3 are higher than the AC voltage that is applied to EL2, so that the EL1 and the EL3 are more brightly luminant than the EL2.
However, if the luminescence brightnesses of a plurality of ELs are individually transfered with the aforementioned methods inverters corresponding in number to ELs will be needed and therefore the structure will be very uneconomical.